Boris Johnson defended a prominent critic of Theresa May as the Tory civil war over Brexit got ugly ahead of crunch Cabinet talks on the UK’s exit strategy.

The Foreign Secretary said MPs should be “able to air their views” after Jacob Rees-Mogg was criticised for saying the Prime Minister must deliver the Brexit she promised or risk collapsing her Government.

The North East Somerset MP warned Tories would vote down any Brexit deal that did not give the UK complete freedom in areas of trade, immigration and fishing.

He said giving in to those MPs who want a soft Brexit would split the party in the same way a row over tariffs on imported corn did in the 19th century.

Despite the principle of Cabinet collective responsibility, Johnson, who was the figurehead of the Leave referendum campaign, appeared more sympathetic to the backbencher than the Prime Minister.

It's vital that all MPs are able to air their views on Brexit. Whatever your position, I hope we can all agree that @Jacob_Rees_Mogg is a principled and dedicated MP who wants the best for our country.

Brexiteer Rees-Mogg, tipped as a potential Tory leader, had used an opinion piece in Monday’s Daily Telegraph to warn the Prime Minister she must deliver what she promised – the UK leaving the single market and customs union and outside the jurisdiction of the European Court of Justice – or risk her administration.

He insisted he was “confident” May would deliver the Brexit she had promised, but warned the PM that backsliding could result in splitting the party like Sir Robert Peel, who plunged it into the political wilderness for nearly three decades following bitter divisions over trade reforms.

The chairman of the European Research Group of Brexit-backing Tories said: “Theresa May must stand firm for what she herself has promised.

“One former Tory leader, Sir Robert Peel, decided to break his manifesto pledge and passed legislation with the majority of his party voting the other way.

“This left the Conservatives out of office for 28 years.

“At least he did so for a policy that works. At Chequers (Mrs May) must stick to her righteous cause and deliver what she has said she would, she must use her undoubted grace to persevere.”

Foreign Office minister Sir Alan said his comments risked “debasing” the Government, Tory Party and the country as a whole, saying: “The ideological right are a minority despite their noise and should pipe down.”

North Dorset MP Simon Hoare said “the hectoring nonsense” and “blackmail” had to stop and urged his party to wake up to the potential “calamity” of a Jeremy Corbyn-led government.

May will bring together her Cabinet at her country residence to thrash out details of a white paper setting out the UK’s plans for areas such as trade.

Brexiteers oppose the PM’s favoured option of a customs partnership with the EU, which would see the UK collect tariffs set by the EU customs union on goods entering the country on behalf of the bloc.

Their “max fac” alternative would, rather than scrapping customs checks, use technology to minimise the need for them. Both options have been dismissed by the EU.

On Monday, May called on EU leaders to show flexibility and look “seriously” at the UK’s Brexit plans.

May said the white paper would set out “detailed proposals for a sustainable and close future relationship” between the UK and EU and mark “an important step in delivering the decision of the British people”.

Addressing MPs in the Commons on Monday afternoon, May again stressed the UK would leave the single market and customs union.

She said: “The EU and its member states will want to consider our proposals seriously.

“We both need to show flexibility to build the deep relationship after we have left that is in the interests of both our peoples.”